Getting Blender to export .ply files with vertex colors
A few things here:

The diffuse color you assigned isn't part of the Vertex Color content, they are distinct from eachother. If you want to give a single colour to all faces, then go to Vertex Paint mode and masking mode, select all faces, then pick a colour from the colour-wheel and press Shift + K . That will set the vertex colour for all faces. and Save.



For triangulation, you might have to make a duplicate of your model and triangulate, then press save, then export. Else I think the exporter will export the mesh's data from the most recent saved state (that wasn't triangulated yet).